This resolves issue #101963.
When the service is started and no interface is ready yet, wpa_supplicant
is being exec'd with no `-i` flags, thus failing. Once the interfaces
are ready, the udev rule would fire but wouldn't restart the unit because
it wasn't currently running (see systemctl(1) try-restart).
The solution is to exit (with a clear error message) but always restart
wpa_supplicant when the interfaces are modified.
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